cc : Fix bug in tree synchronization when mask layer changes

cc/trees/layer_tree_host_unittest.cc

Index: cc/trees/layer_tree_host_unittest.cc
diff --git a/cc/trees/layer_tree_host_unittest.cc b/cc/trees/layer_tree_host_unittest.cc
index cee77ff3d34e2ff457c963fbb93e9866c388edb1..535ea3e9ace3b2b8a3f885128ed2f01db6079927 100644
--- a/cc/trees/layer_tree_host_unittest.cc
+++ b/cc/trees/layer_tree_host_unittest.cc
@@ -780,6 +780,53 @@ class LayerTreeHostTestPropertyTreesChangedSync : public LayerTreeHostTest {
 
 SINGLE_THREAD_TEST_F(LayerTreeHostTestPropertyTreesChangedSync);
 
+class LayerTreeHostTestSwitchMaskLayer : public LayerTreeHostTest {
+ protected:
+  void SetupTree() override {
+    scoped_refptr<Layer> root = Layer::Create();
+    scoped_refptr<Layer> child = Layer::Create();
+    mask_layer = Layer::Create();
+    child->SetMaskLayer(mask_layer.get());
+    root->AddChild(std::move(child));
+    layer_tree_host()->SetRootLayer(root);
+    LayerTreeHostTest::SetupTree();
+  }
+
+  void BeginTest() override {
+    index_ = 0;
+    PostSetNeedsCommitToMainThread();
+  }
+
+  void DidCommit() override {
+    switch (layer_tree_host()->source_frame_number()) {
+      case 1:
+        layer_tree_host()->root_layer()->RemoveAllChildren();
+        layer_tree_host()->root_layer()->SetMaskLayer(mask_layer.get());
+        break;
+    }
+  }
+
+  void CommitCompleteOnThread(LayerTreeHostImpl* impl) override {
+    switch (index_) {
+      case 0:
+        index_++;
+        EXPECT_FALSE(impl->active_tree()->root_layer()->mask_layer());
+        break;
+      case 1:
+        EXPECT_TRUE(impl->active_tree()->root_layer()->mask_layer());
+        EndTest();
+        break;
+    }
+  }
+
+  void AfterTest() override {}
+
+  scoped_refptr<Layer> mask_layer;
+  int index_;
+};
+
+SINGLE_THREAD_TEST_F(LayerTreeHostTestSwitchMaskLayer);

[ajuma, 2016/03/30 21:50:59]

Any reason for this to be single-thread only? (If we change references to
active_tree to sync_tree, does the test still work multi-threaded?)

[danakj, 2016/03/30 22:13:35]

Thanks for asking this, would also appreciate a comment on top of the macro
explaining why, we should have this on every test that diverges from
MULTI_AND_SINGLE.

[jaydasika, 2016/03/30 22:33:34]

Changed it to single and multi thread.

+
 // 1 setNeedsRedraw after the first commit has completed should lead to 1
 // additional draw.
 class LayerTreeHostTestSetNeedsRedraw : public LayerTreeHostTest {